DXing Digital Utilities
NOTICE: The rules about listening to signals not intended for you applies here. The contents of many signals might be considered sensitive by the party sending and the reception of such signals may be illegal in your country. The authors neither condone or encourage such acts
This document is designed to give utility listeners a sampling of the kinds of signals and sounds available on shortwave (HF) radio today along with information on the available equipment needed to understand, analyze or identify these signals. Our primary focus is to present the parameters that define the most commonly heard systems as an aid for utility monitors and not an exhaustive discussion of HF
digital signaling theory.
Digital Utility listening is inherently more difficult than regular shortwave listening. The possibility of decoding the signal received adds another level of complexity. This document is an attempt to describe to hobbyists what digital signals can be received and decoded, in addition to providing the more advanced listener with a little more technical data to identify an otherwise unknown signal.
Herein lies the basic problem with digital utility listening - lack of information. Many systems are used by Military or Diplomatic Services and information on the specifics of a particular mode are impossible to find, even from the manufacturer. Many are considered proprietary, but that doesn't mean that a signal can not be identified. With the proper tools a given signal can be identified via the way it sounds (aurally) or how it looks (visually). Most of the more expensive decoders that include some kind of signal analysis can ID a signal by bit-pattern or baud rate. Once a signal is identified there are many decoders that can print the traffic for you; however, keep in mind there may be various kinds of encryption in use with these signals. Encryption types include figure group or letter group messages and even random bit-masking or bit stream encryption, which looks like a continuous stream of random characters. You may often read the term "on-line" and "off-line" used in conjunction with various encryption schemes. Generally, off-line encryption is taken to mean groups of letters or numbers (most usually groups of five), whereas on-line schemes just appear as a continuous stream of random characters.
Keep in mind that you must be able to find a signal before you can apply the power of the decoder on the signal for identification and possible decoding. Most signals found on the airwaves today are obvious with easily distinguishable sounds, from chirping to two tone FSK to musical multitone MFSK, but as communication technology develops this will most likely change. It is safe to say that the more efficient a modulation/coding method is, the more noise like it must become. It has been said in some digital groups that "Any sufficiently advanced communication is indistinguishable from noise".
This document covers the following areas. Please click on the desired topic;
- Chapter 1. Receivers
- Chapter 2. Decoding Software
- Chapter 3. Modes and Sounds
- Chapter 4. Articles
- Chapter 5. Scheduled and Easy to Hear Stations
- Chapter 6. References